Natural gas is generally sold by volumetric measurement, and one way to measure the volume of a flowing gas is to pass it through an orifice of a known size and measure the pressure drop across it. An orifice meter comprises a housing within which a disc with an orifice is supported in a gas pipeline, the housing being fitted with pressure taps for measuring pressure immediately upstream and downstream of the orifice. Because the orifice is subject to wear by impacting of sand, lime scale and other foreign particles in the flowing stream, it must be replaced at frequent intervals to ensure accuracy in measurement.
In the more sophisticated type orifice meters, the orifice disc carrier is moved to a location in the housing which may be sealed off from the flow passage so that flow can continue while the orifice disc is being replaced. A closure covering an access opening in a wall of the meter housing is removed so to enable service or replacement of the orifice disc. However, if the closure is removed or loosened while the body compartment is still under pressure, serious or even catastrophic consequences may ensue.